Thermal behavior and dissociation efficiency of methane hydrate during depressurization near the quadruple point

During methane hydrate production by depressurization, rapid temperature decline, insufficient heat supply, and heat-transfer-limited dissociation are commonly encountered. These issues become particularly pronounced near the quadruple point, where the dissociation pathway may deviate from the conventional route above the ice-freezing point. In this study, quartz sand was used as the porous medium at an ambient temperature of 2 °C, and four depressurization paths, namely Run.1, Run.2, Run.3, and Run.4, were designed. The coupled evolution of pressure and temperature, hydrate dissociation ratio, dissociation rate, heat flux, cumulative heat absorption, and the spatiotemporal variation of the two-dimensional temperature field were systematically analyzed during dissociation. The results show that, compared with Run.1, the time required for Run.2 to reach 90% dissociation decreased from 7.20 h to 4.61 h, whereas the cumulative heat absorption remained at a comparable level. This indicates that the advantage of Run.2 mainly lies in the improved utilization efficiency of heat during dissociation, rather than in a substantial reduction in total heat consumption. After the pressure was further reduced to 1 MPa, the cumulative heat absorption at 90% dissociation decreased to 346.34 kJ and 180.04 kJ for Run.3 and Run.4, respectively, indicating that dissociation pathways below the ice-freezing point can significantly reduce the overall heat demand. Combined with the pressure-temperature evolution trajectories, heat-flux responses, and spatiotemporal variations in the temperature field, it is inferred that Run.3 may have undergone a thermally regulated evolution process controlled by a metastable stage, involving metastable buffering, subsequent destabilization, local ice formation, and local heat release that promoted dissociation. These findings suggest that the pressure node near the quadruple point should not be regarded merely as an operational pressure level, but rather as a critical thermal control window for regulating heat allocation, heat-transfer efficiency, and the persistence of hydrate dissociation.